Carbon-rich coatings can be quite hard, chemically inert, corrosion resistant, and impervious to water vapor and oxygen. Thus, they are often used as mechanical and chemical protective coatings on a wide variety of substrates. For example, carbon-rich coatings have been applied to rigid disks and flexible magnetic media. They have also been applied to acoustic diaphragms, polymeric substrates used in optical and ophthalmic lenses, as well as electrostatic photographic drums.
Carbon-rich coatings, as used herein, contain at least 50 atom percent carbon, and typically about 70-95 atom percent carbon, 0.1-20 atom percent nitrogen, 0.1-15 atom percent oxygen, and 0.1-40 atom percent hydrogen. Such carbon-rich coatings can be classified as "amorphous" carbon coatings, "hydrogenated amorphous" carbon coatings, "graphitic" coatings, "i-carbon" coatings, "diamond-like" coatings, etc., depending on their physical and chemical properties. Although the molecular structures of each of these coating types are not always readily distinguished, they typically contain two types of carbon-carbon bonds, i.e., trigonal graphite bonds (sp.sup.2) and tetrahedral diamond bonds (sp.sup.3). They can also contain carbon-hydrogen bonds and carbon-oxygen bonds, etc. Depending on the amount of noncarbon atoms and the ratio of sp.sup.3 /sp.sup.2 bonds, different structural and physical characteristics can be obtained.
Diamond-like carbon-rich coatings have diamond-like properties of extreme hardness, extremely low electrical conductivity, low coefficients of friction, and optical transparency over a wide range of wavelengths. They can be hydrogenated or nonhydrogenated. Diamond-like carbon coatings typically contain noncrystalline material having both trigonal graphite bonds (sp.sup.2) and tetrahedral diamond bonds (sp.sup.3); although it is believed the sp.sup.3 bonding dominates. Generally, diamond-like coatings are harder than graphitic carbon coatings, which are harder than carbon coatings having a large hydrogen content, i.e., coatings containing hydrocarbon molecules or portions thereof.
Methods for preparing coatings by plasma deposition, i.e., plasma-assisted chemical vapor deposition, are known; however, some of these methods have deficiencies. For example, with certain methods the use of high gas flow, pressure, and power can cause formation of carbon powder, instead of the desirable smooth, hard carbon film. U.S. Pat. Nos. 5,232,791 (Kohler et al., Aug. 3, 1993) and 5,286,534 (Kohler et al., Feb. 15, 1994) disclose a process for the plasma deposition of a carbon-rich coating that overcomes some of these deficiencies. This process uses a carbon-rich plasma, which is generated from a gas, such as methane, ethylene, methyliodide, methylcyanide, or tetramethylsilane, in an elongated hollow cathode, i.e., a tubular cathode typically having a length to diameter ratio of 15:1 to 1:1. The plasma is accelerated toward a substrate exposed to a radio frequency bias. Although this process represents a significant advancement in the art, other plasma deposition processes are needed for deposition of a wide variety of carbon-rich coatings using lower energy requirements.